Expose subagent token usage in status line

Problem

The status line's token count (total_input_tokens / total_output_tokens) only tracks the main conversation. Subagent tokens (Agent tool) are invisible — they run in isolated contexts with no metrics surfaced to the status line JSON.

On Max plans, /cost only confirms "using your subscription" — no token or dollar breakdown.

Proposed solution

Add subagent token fields to the status line JSON:

{
  "context_window": {
    "total_input_tokens": 10000,
    "total_output_tokens": 2000,
    "subagent_total_input_tokens": 45000,
    "subagent_total_output_tokens": 8000
  }
}

Enabling status lines like:

project (main) | Opus 4.6 | 3% ctx | 12k + 53k tok

Where 12k = main conversation, 53k = cumulative subagent tokens.

The data already exists in ~/.claude/projects/{project}/{sessionId}/subagents/agent-{agentId}.jsonl — it just needs aggregation into the status line schema.

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Drafted by Claude Code (Opus 4.6), reviewed by author.